Mass spectrometers which employ an ion trap to trap (confine) ions by means of an AC electric field are known in the prior art. A typical ion trap is a so-called three-dimensional quadrupole ion trap comprising a ring electrode of a substantially circular ring shape, and a pair of end cap electrodes arranged so as to sandwich the ring electrode. Generally, in such ion traps, a trapping electric field is formed in the space surrounded by the electrodes by applying a sinusoidal high frequency voltage to the ring electrode, and ions are vibrated and confined by means of this trapping electric field.
In an ion trap mass spectrometer, after trapping the ions to be analyzed in an ion trap, the mass separation function of the ion trap itself is used to selectively eject ions having a specified mass-to-charge ratio from the ion trap, and their ion intensity is detected by means of a detector provided outside the ion trap. Furthermore, in an ion trap time-of-flight mass spectrometer, after trapping the ions to be analyzed in an ion trap, kinetic energy is imparted to the ions and the ions are released at once from the ion trap and injected into a time-of-flight mass spectrometer, in which the ions are separated and detected according to their mass-to-charge ratios. In both cases, in order to detect ions with high sensitivity, it is necessary for ions generated in an outside ion source to be efficiently injected into an ion trap.
To trap ions inside an ion trap, a high frequency voltage is applied to the ring electrode to form a high frequency electric field as described above, but this high frequency electric field becomes a potential barrier for ions trying to enter from outside. Thus, when one attempts to inject ions into the ion trap from outside in a state where a high frequency electric field has been formed, the ions will be bounced back by the aforementioned potential barrier or will rather end up being overly accelerated. As a result, the efficiency of ion injection into an ion trap in a state where a high frequency electric field has been formed was at most on the order of several percent.
To resolve this, in the method described in Patent Literature 1, control is performed whereby the application of high frequency voltage to the ring electrode is temporarily stopped when injecting packetized ions into an ion trap, and is restarted immediately after the ions have been injected into the ion trap. It should be noted that when the sample ionization technique is MALDI, since the sample is irradiated with a pulsed laser light, ions are generated in the form of packets at short intervals, while when an atmospheric ion source is used, such as in the electrospray ionization (ESI) method, ions are generated continuously, so in the device described in Patent Literature 1, the ions are held back in the ion guide and allowed to build up for a while before ejecting them at once from the ion guide, thereby turning a continuous ion stream into packet-like ion groups.
In the aforementioned method, the injection of ions into the ion trap is not hindered by the high frequency electric field, and the ion injection efficiency can be increased. However, when there is no high frequency electric field, no trapping effects on ions is produced, so it is necessary to start the application of high frequency voltage to the ring electrode before the ions that were injected into the ion trap spread out too much. Therefore, in order to properly trap the ions injected into the ion trap, the timing of application of high frequency voltage is important, and that timing needs to be properly adjusted in order to optimize the ion injection efficiency.
In recent years, an ion trap with a digital drive system which performs ion confinement by applying square wave voltage to the ring electrode instead of sine wave voltage—the so-called digital ion trap (DIT)—has been developed (see Non-Patent Literature 1, etc.).
In a DIT, square wave high voltage is generated by switching DC high voltage by means of semiconductor switch and the like, so it has the characteristic of allowing frequency modification of the square wave high voltage, duty ratio control, phase control and the like to be easily performed. Patent Literature 2 describes performing control in a mass spectrometer equipped with such as DIT, whereby, in a state where ions are held inside the ion trap, to additionally inject ions into the ion trap, the application of square wave voltage to the ring electrode is temporarily stopped and is restarted after ion injection. It is disclosed that, in this case, it is necessary to reliably capture the additionally injected ions while preventing the dissipation of ions already trapped in the ion trap, and that a time of 1 to 50 μsec is suitable as the time for stopping the application of square wave voltage.